Clock synthesizers, which are often implemented as phase-locked loop (PLL) circuits, may be used to generate high-frequency signals based on a lower frequency reference signal. Many PLLs include a voltage-controlled oscillator (VCO) to generate an output signal at the desired frequency. For example, a VCO generates an oscillating signal having a frequency of oscillation that is controlled by a voltage provided to the VCO (e.g., a control voltage). However, changes in temperature and other variations in the operating conditions of the VCO and/or PLL may cause variations in the frequency of the output signal. Thus, the control voltage may be adjusted to compensate for changes in operating conditions.
Inductor-capacitor (LC) VCOs tend to be especially sensitive to process, voltage, and temperature (PVT) variations. Such variations in process and voltage are typically compensated for when selecting a particular carrier frequency for a given application. However, the temperature of the PLL and/or VCO may continually change over time. Changes in temperature may affect the ability of the VCO to maintain the output signal at the desired frequency of oscillation. For example, a significant change in temperature may require an equally significant change in the control voltage of the VCO to maintain the output signal at the desired frequency. If the change in control voltage should exceed the dynamic range of the PLL (or VCO), the VCO may need to be reset. Resetting the VCO may cause the PLL to break lock.
Thus, it is desirable to compensate for changes in operating conditions (e.g., temperature) of the VCO while ensuring that the control voltage remains within the dynamic range of the VCO.